The present invention relates to rotary fluid pressure devices of the type in which a gerotor gear set serves as the fluid displacement mechanism, and more particularly, to such devices which are provided with two speed capability.
Although the teachings of the present invention can be applied to devices having fluid displacement mechanisms other than gerotors, such as cam lobe type devices, the invention is especially adapted to gerotor devices and will be described in connection therewith.
Devices utilizing gerotor gear sets can be used in a variety of applications, one of the most common being to use the device as a low-speed, high-torque motor. One common application for low-speed, high-torque gerotor motors is vehicle propulsion, wherein the vehicle includes an engine driven pump which provides pressurized fluid to a pair of gerotor motors, with each motor being associated with one of the drive wheels. Those skilled in the art will be aware that many gerotor motors utilize a roller gerotor, especially on larger, higher torque motors of the type used in propel applications, and subsequent references hereinafter to "gerotors" will be understood to mean and include both conventional gerotors, as well as roller gerotors.
In recent years, there has been a desire on the part of the vehicle manufacturers to be able to provide both the low-speed, high-torque mode of operation, such as when the vehicle is at the work site, and also a high-speed, low-torque mode of operation, for when the vehicle is traveling between work sites. One possible solution has been to provide a gerotor motor having a two-speed capability.
Two speed gerotor motors are known from U.S. Pat. No. 4,480,971, assigned to the assignee of the present invention and incorporated herein by reference. The device of the cited patent has been in widespread commercial use and has performed in a generally satisfactory manner. As is well known to those skilled in the art, a gerotor motor may be operated as a two speed device by providing valving which can effectively "recirculate" fluid between expanding and contracting fluid volume chambers of the gerotor gear set. In other words, if the inlet port communicates with all of the expanding chambers, and all of the contracting chambers communicate with the outlet port, the motor operates in the normal low-speed, high-torque mode. If some of the fluid from the contracting chambers is recirculated back to some of the expanding chambers, the result will be operation in a high-speed, low-torque mode.
However, one of the inherent shortcomings of the design of the cited patent is that the valving has been of the "three zone" type, i.e., there is one zone communicating with the inlet, one zone communicating with the outlet, and one changeover zone. As a result of this three zone architecture, when the motor operates in the clockwise direction, for example, high pressure fluid is recirculated, but when the motor operates in the counterclockwise direction, low pressure fluid is recirculated. As is also well known to those skilled in the art, recirculation of low pressure fluid can result in cavitation within the valving and the gerotor gear set, and such cavitation can eventually lead to failure of the motor.
Another problem with the device of the cited patent is that the configuration of the balancing ring was such that several seals were required at various locations on several outside diameters of the balancing ring, sealing between the ring and adjacent inside diameters of the valve housing of the motor. This type of multiple-diameter sealing added to the difficulty and expense of the machining and assembly of the motor.
There has been a device commercially available in which the valving provided four zones, such that the two middle zones are connected to the recirculating volume chambers in such a way that high pressure is always recirculated, for either direction of operation. The described device, commercialized by Sumitomo Eaton Hydraulics Co., Ltd., a licensee of the assignee of the present invention, had the valving located "forward" of the gerotor, i.e., between the gerotor gear set and the output shaft. The valving configuration is such that the overall package is quite large, the associated control valving, to shift between low-speed and high-speed, is quite complicated, and the resulting motor would not be commercially acceptable for many applications.
One additional problem associated with two speed gerotor motors is that the shifting between low-speed, high-torque and high-speed, low-torque is typically somewhat abrupt or harsh, resulting in a sudden acceleration or deceleration of the vehicle. Naturally, the vehicle operators would prefer that the shifting between the two modes of operation be smooth, rather than too quick or too harsh, as shifts which are too quick can result in tipping of the vehicle or losing control of a load, such as a load on the tines of a forklift truck.
A final problem associated with vehicles equipped with two speed gerotor motors is that certain vehicles are equipped with a pair of motors to drive a pair of propel wheels in a parallel circuit. On such a vehicle, it has been difficult to get the motors to shift at the same time. However, if there is a delay between the shifting of one motor and the shifting of the other motor, the result will be an inadvertent turning of the vehicle during the time one motor is operating at high-speed and the other motor is operating at low-speed.